Leaf curl symptoms that are reminiscent of begomovirus (genus Begomovirus, family Geminiviridae) infection were observed widespread in the tomato crop during the early fall 2005 through the spring 2006 growing seasons in Sinaloa State, Mexico. Symptoms were widespread in three major valleys (Culiacan, Guasave, and Los Mochis) that are largely dedicated to fresh-market tomato production for the U.S. market from October to June. Symptoms included stunting of leaves, shortened internodes, distortion of leaf margins, and green vein banding. Fruit set was reduced significantly (as much as 90%) on the portion of the plant that developed above the point of symptom expression. Tomato fields were heavily infested with the B biotype of the whitefly Bemisia tabaci (Genn.) vector and no other insect vectors were noted in the fields. Total DNA was extracted from six symptomatic tomato plants (two from each valley) and used as template to amplify, clone, and sequence the core region of the begomovirus CP. BLAST analysis of begomovirus sequences available in the NCBI GenBank database indicated the closest match was the Old World monopartite begomovirus Tomato yellow leaf curl virus (TYLCV) from Israel (Accession No. X15656) at 97.8% shared nucleotide (nt) identity. The full-length genome was amplified for each of six isolates using TempliPhi (Amersham Biosciences, Piscataway, NJ) and cloned into the pGEM7 vector (Promega, Madison, WI). The complete DNA genome sequence was determined for eight clones by primer walking. Cloned TempliPhi products sequenced represented two to three isolates from each valley. Results indicated that the isolates (n = 8) were 98.9 to 100% identical (Accession No. DQ631892) to each other, and they shared 98% identity with TYLCV isolates reported from the Caribbean Region and Florida. This highly virulent begomovirus of tomato, originating in Israel, was first reported in Mexico from 1996 to 1997 when it was identified in tomato plants in the Yucatan Peninsula (1) (>1,500 miles from Sinaloa). The latter report followed the introduction of TYLCV in tomato seedlings from Florida into several eastern U.S. states (3,4) and then into Puerto Rico (2). The introduction of TYLCV into Sinaloa where tomato production is highly concentrated is significant because the region supplies the majority (as much as 93%) of fresh-market tomatoes to the western United States from October to June (>$750 million dollars). Of equal importance is the immediate proximity of the pandemic to California where more than 90% of the processing tomatoes in the United States are grown.